How did this 'ghost plane' cross the US unnoticed?

On October 25, an unidentified aircraft was seen flying - in broad daylight - among the airliners in one of the United States' heavily trafficked air corridors in the skies above Oregon.

It had no submitted flight plan. It had no identification transponder active. Nor was it transmitting collision avoidance signals.

Air traffic control stations were reportedly having difficulty following it on radar.

Coming after the events of 9/11, such an unidentified aircraft is not just a matter for UFO cranks and conspiracy theorists.

It represents a potentially serious breach of national security.

Or does it?

The War Zone blog of automotive news service The Drive began digging for an answer after one of its reporters heard scuttlebut about the incident from aviation industry associates.

At first it sounded like a typical example of an aircraft suffering communications failure.

But then came news, via Reddit, that US air force F-15C interceptor fighters had been launched in response to the sightings.

"Strange! My theory is they were running drugs to Canada. No news yet, not that I could find," a user claiming to have been a pilot in the air at the time posted user 'Duprass'.

Two F-15 jet fighters in flight over Crater Lake, Oregon. Despite their speed and radar, they were reportedly unable to locate the mystery aircraft. Picture: USAFSource:News Corp Australia

He described how airliners had been asked to help track an unknown white-coloured aircraft visually for up to 30 minutes as air traffic control radar was having difficulty getting a fix.

While it was in sight of various airliner crews, it was apparently never close enough for its type to be identified.

"The last aeroplane to see it had to descend into Portland and lost sight of it. The fighters were scrambled out of PDX but flew around for a while and did not find it. And that's that."

War Zone contacted North American Aerospace Defence Command (NORAD), the 142nd Fighter Wing at Portland International Airport, and the Federal Aviation Authority (FAA) to confirm the incident.

NORAD quickly confirmed it had been asked by the FAA to help track an unidentified aircraft flying at heights regularly used by commercial aircraft (35,000 and 40,000ft). It said fighters had been scrambled from Portland to investigate - but failed to find anything.

This is in itself odd.

While little is known about the timing of the events (it is possible the fighters were simply activated too late), the F-15C Eagle interceptors have some of the most advanced combat search and tracking radars in the world.

They're also extremely fast.

So while the unidentified aircraft was reported to have been moving somewhat faster than the observing airliners, a F-15 should have been able to catch up easily.

War Zone says the FAA has refused to add any detail to the story other than to confirm what had already been established. The 142nd Fighter Wing did not respond.

So what was it?

That remains speculation.

Oregon is adjacent Nevada, the well-known home to the United States Air Force's secret aircraft testing facility at Groom Lake (otherwise dubbed Area 51). But flying a secret aircraft among commercial airliner streams in daylight is both dangerous and insecure - and not known to be a common practice.

Was it Russian. Was it Chinese?

It may have been a smuggling aircraft, its cashed-up owners giving it an added dimension of speed and stealth. But there is no evidence to suggest such an aircraft exists.